sagemathgh-36456: implement .interpolation() method for ProductTree
    
The `ProductTree` class has a `.remainders()` method which can, among
other things, be used to implement the Fast Fourier Transform. In this
patch we add a corresponding `.interpolation()` method, which can, among
other things, be used to implement the *inverse* Fast Fourier Transform.
Its functionality is equivalent to `CRT_list()`, but caching the
product-tree structure makes it significantly faster for repeated
invocations.
    
URL: sagemath#36456
Reported by: Lorenz Panny
Reviewer(s): Kwankyu Lee

Author: Release Manager

src/sage/rings/generic.py

@@ -28,10 +28,17 @@ class ProductTree:
         sage: R.<x> = F[]
         sage: ms = [x - a^i for i in range(1024)]               # roots of unity
         sage: ys = [F.random_element() for _ in range(1024)]    # input vector
-        sage: zs = ProductTree(ms).remainders(R(ys))            # compute FFT!
+        sage: tree = ProductTree(ms)
+        sage: zs = tree.remainders(R(ys))                       # compute FFT!
         sage: zs == [R(ys) % m for m in ms]
         True
 
+    Similarly, the :meth:`interpolation` method can be used to implement
+    the inverse Fast Fourier Transform::
+
+        sage: tree.interpolation(zs).padded_list(len(ys)) == ys
+        True
+
     This class encodes the tree as *layers*: Layer `0` is just a tuple
     of the leaves. Layer `i+1` is obtained from layer `i` by replacing
     each pair of two adjacent elements by their product, starting from
@@ -177,7 +184,6 @@ def remainders(self, x):
         The base ring must support the ``%`` operator for this
         method to work.
 
-
         INPUT:
 
         - ``x`` -- an element of the base ring of this product tree
@@ -199,6 +205,55 @@ def remainders(self, x):
             X = [X[i // 2] % V[i] for i in range(len(V))]
         return X
 
+    _crt_bases = None
+
+    def interpolation(self, xs):
+        r"""
+        Given a sequence ``xs`` of values, one per leaf, return a
+        single element `x` which is congruent to the `i`\th value in
+        ``xs`` modulo the `i`\th leaf, for all `i`.
+
+        This is an explicit version of the Chinese remainder theorem;
+        see also :meth:`CRT`. Using this product tree is faster for
+        repeated calls since the required CRT bases are cached after
+        the first run.
+
+        The base ring must support the :func:`xgcd` function for this
+        method to work.
+
+        EXAMPLES::
+
+            sage: from sage.rings.generic import ProductTree
+            sage: vs = prime_range(100)
+            sage: tree = ProductTree(vs)
+            sage: tree.interpolation([1, 1, 2, 1, 9, 1, 7, 15, 8, 20, 15, 6, 27, 11, 2, 6, 0, 25, 49, 5, 51, 4, 19, 74, 13])
+            1085749272377676749812331719267
+
+        This method is faster than :func:`CRT` for repeated calls with
+        the same moduli::
+
+            sage: vs = prime_range(1000,2000)
+            sage: rs = lambda: [randrange(1,100) for _ in vs]
+            sage: tree = ProductTree(vs)
+            sage: %timeit CRT(rs(), vs)             # not tested
+            372 µs ± 3.34 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
+            sage: %timeit tree.interpolation(rs())  # not tested
+            146 µs ± 479 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)
+        """
+        if self._crt_bases is None:
+            from sage.arith.misc import CRT_basis
+            self._crt_bases = []
+            for V in self.layers[:-1]:
+                B = tuple(CRT_basis(V[i:i+2]) for i in range(0, len(V), 2))
+                self._crt_bases.append(B)
+        if len(xs) != len(self.layers[0]):
+            raise ValueError('number of given elements must equal the number of leaves')
+        for basis, layer in zip(self._crt_bases, self.layers[1:]):
+            xs = [sum(c*x for c, x in zip(cs, xs[2*i:2*i+2])) % mod
+                  for i, (cs, mod) in enumerate(zip(basis, layer))]
+        assert len(xs) == 1
+        return xs[0]
+
 
 def prod_with_derivative(pairs):
     r"""